Optical fiber communications systems, employing fiber optic cables and digital electronics, are widely used in the telecommunication industry to transmit large volumes of data and voice signals over relatively long unrepeatered distances, and virtually noise free. Splice points and drop points for the fiber optic cables are required for most such systems. At a splice point, for example, all of the fibers at one end of a cable are spliced to corresponding fibers of a tandem cable. At a drop point or express splice point, some of the fibers may be spliced to a drop cable, while most of the fibers are passed through the drop point unaltered.
For both splice points and drop points, the optical fibers are exposed from the protective cable jacket to be spliced and secured within a splice closure. The splice closure typically includes a protective housing with either a single end cap through which cables penetrate, that is, a butt-splice; or dual opposing end caps through which respective cables penetrate, that is, an in-line splice.
A typical butt-splice closure, such as the model FOSC 100 made by the assignee of the present invention, typically includes one or more splice organizers, or splice trays, disposed in stacked arrangement within the protective housing. The trays are pivotally connected at one end to a mounting bracket which, in turn, is connected to the inside face of the closure end cap. The pivotal connection permits individual splice trays to be temporarily moved to a raised position by the insertion of a removable spacer or clip near the pivot point. Accordingly, access is then available to the underlying splice tray, such as to check fiber routing or to remake a defective splice.
The cables extending into the housing are secured therein and the penetration point sealed to prevent water from entering the protective housing. Since the protective cable sheath is removed within the housing, flexible protective tubes, known as "transport tubes" are used to protect predetermined groupings of the optical fibers extending from the cable securing point to respective splice organizer trays. Such conventional transport tubes typically have a circular cross-section and a uniform wall thickness. For a typical fiber optic cable of the loose-buffer type, the predetermined groupings are typically all those fibers within a given buffer tube. In other words, a transport tube slides over an end portion of a respective buffer tube to carry and protect the fibers extending to the splice tray. One or more such transport tubes are routed to and secured to each splice tray.
The transport tubes must protect the optical fibers despite any bending that occurs, such as when the splice trays are pivoted to the raised position to access an underlying tray. Moreover, the transport tubes must prevent the optical fibers from bending more sharply than the minimum bend radius. Since individual fibers extend through the transport tubes when using a loose buffered cable, these individual fibers may readily bend along with the transport tube.
For many applications, higher fiber count cables are required. Higher fiber count cables having a relatively small cable cross-section are available and include a plurality of optical fiber ribbons, such as LIGHTPACK.RTM. fiber optic cables offered by AT&T. Optical fiber ribbons may be readily bent only in a direction normal to their major dimension equivalently to the minimum bend radius of the individual fibers. However, the ribbons may not be bent as sharply in the direction normal to their minor dimension. In other words, optical fiber ribbons preferentially bend only in the direction normal to their major dimension. Moreover, the ribbons should not be bent in the direction of their minor dimension or high signal attenuation or physical damage may result.
Unfortunately, optical fiber ribbons positioned within a conventional transport tube may be bent in any direction thereby increasing attenuation and possibly physically damaging the ribbons. In addition, a conventional transport tube also permits an optical fiber ribbon to be deformed from its flat shape and compressed or buckled when a tie wrap, for example, is used to secure the end of the transport tube to a splice organizer tray. Accordingly, a conventional circular cross-sectional transport tube is unacceptable for use in a splice closure for ribbon optical fiber cables.
Also related to the quality and longevity of optical fiber splices secured within a splice closure is a splice holder, several of which are typically mounted on a splice tray. The splice holder retains the individual splices between corresponding optical fibers. A typical splice holder may accommodate four to ten splices and must adequately secure the splices in the presence of mechanical shocks and vibration. The splices are typically protective sleeves for fusion-spliced fibers, or may be mechanical splices which position and maintain the optical fiber ends in precise alignment. For example, U.S. Pat. No. 4,679,896 to Krafcik et al. discloses a typical splice holder formed of a resilient block with a series of channels formed therein to closely resiliently receive optical fiber splices.
There is no industry standard for the precise external dimensions of an optical fiber splice; rather, there are a number of popular commercially available mechanical and fusion splices, most with different exterior dimensions. AT&T in an attempt to accommodate a number of different types of splices of different sizes, for example, offers a splice holder having a series of spaced apart deformable walls of a foam material to accommodate different sized splices. In a similar fashion, U.S. Pat. No. 4,793,681 to Barlow et al. discloses a splice holder with pairs of opposing leaf springs to accommodate different sized splices. U.S. Pat. No. 4,854,661 to Cooper et al. discloses a lid over the splice holder with a resilient pad positioned within the lid to hold the fiber splices within respective shallow grooves of the underlying splice holder. Similarly the DeSanti patent, U.S. Pat. No. 4,687,289, includes a lid which may be offset from underlying grooves to thereby accommodate a slightly smaller fusion splice, as compared to a typical mechanical splice. Despite attempts to accommodate different sized splices there still exists a need to do so while properly cushioning the splices against mechanical shock and vibration.
A splice closure also typically includes a lower slack storage tray adjacent the stacked splice trays. The slack storage tray is particularly important for a drop splice where a large number of fibers are passed through the splice closure without being spliced. A typical slack storage tray is generally rectangular in shape with perpendicularly extending opposing sidewalls. The slack storage tray extends generally lengthwise within a cylindrical housing as in the FOSC 100 splice closure. Unfortunately, such a storage tray has a limited capacity for slack storage because the height of its perpendicularly extending sidewalls is limited by the size of the cylindrical housing. As higher fiber count cables are required, especially for drop or express splice points, additional slack storage capacity is needed.
Another concern relating to splice closures includes an ability to preferentially separate a desired optical fiber or ribbon from a slack bundle on a splice organizer tray with minimum disturbance to adjacent fibers. This tedious task is typically attempted by using a relatively small hooked probe, such as a crochet needle, to separate a desired optical fiber or ribbon from slack which is positioned adjacent side walls of the splice tray.